Heretofore, a fluorinated copolymer comprising tetrafluoroethylene and propylene has been applied to severe environment which a conventional rubber material cannot withstand, as a rubber material very excellent in the heat resistance and the chemical resistance.
In recent years, utilizing its voltage resistance, oxidation resistance and chemical resistance, it has been developed as a binder polymer for producing an electrode for a storage battery device such as a capacitor, a primer battery and a secondary battery for electronic devices or electric cars, for which a high output, a high capacity and excellent cycle properties are required.
As a binder to be used for producing an electrode for a storage battery device (hereinafter referred to as a binder for producing an electrode), mainly an organic solvent binder having a binder polymer dissolved in an organic solvent or an aqueous binder having a binder polymer dissolved or dispersed in water is used. Since an organic solvent binder which contains an organic solvent has a high environmental burden and is problematic in view of work environment, an aqueous binder particularly attracts attention in recent years.
Patent Document 1 discloses a method for preparing a paste for electrode coating without using a solvent, by using an aqueous dispersion having a copolymer of propylene and tetrafluoroethylene emulsified or dispersed in water. However, it failed to disclose a method for producing such a copolymer.
Patent Document 2 discloses, as a method of copolymerizing propylene and tetrafluoroethylene in an aqueous medium, a method of carrying out a copolymerization reaction in the presence of an oxidation-reduction catalyst (a so-called redox catalyst) comprising a water-soluble persulfate, a water-soluble iron salt, a hydroxymethane sulfinate, and ethylenediamine tetraacetic acid or its salt.
However, for a sealing material of a semiconductor production apparatus and for a material for an electronic industry such as a binder for producing an electrode of a secondary battery, for which a low content of metal components is required, a method of using a redox catalyst including iron is not favorable.
Particularly if a binder for producing an electrode of a secondary battery contains a metal component such as iron, copper or sodium, the initial capacity of the battery may deteriorate, or the performance may be lowered due to deposition on an electrode, and it is required to reduce the content of metal components as far as possible.
Patent Document 3 discloses a method of producing a fluorinated copolymer latex by emulsion polymerization in an aqueous medium, recovering a polymer from the latex and use it as a material for a crosslinked fluorinated rubber. It discloses that in the emulsion polymerization step, the fluorinated copolymer latex can be produced utilizing a reaction by thermal decomposition of a thermally decomposable radical initiator, even without employing a reaction by a redox catalyst.